block: Introduce blk_set_allow_write_beyond_eof()

We check that the guest can't write beyond the end of its disk, but for
other internal users it can make sense to allow growing a file.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/block-backend.c

@@ -50,6 +50,8 @@ struct BlockBackend {
     bool iostatus_enabled;
     BlockDeviceIoStatus iostatus;
 
+    bool allow_write_beyond_eof;
+
     NotifierList remove_bs_notifiers, insert_bs_notifiers;
 };
 
@@ -579,6 +581,11 @@ void blk_iostatus_set_err(BlockBackend *blk, int error)
     }
 }
 
+void blk_set_allow_write_beyond_eof(BlockBackend *blk, bool allow)
+{
+    blk->allow_write_beyond_eof = allow;
+}
+
 static int blk_check_byte_request(BlockBackend *blk, int64_t offset,
                                   size_t size)
 {
@@ -592,18 +599,20 @@ static int blk_check_byte_request(BlockBackend *blk, int64_t offset,
         return -ENOMEDIUM;
     }
 
+    if (offset < 0) {
+        return -EIO;
+    }
+
+    if (!blk->allow_write_beyond_eof) {
         len = blk_getlength(blk);
         if (len < 0) {
             return len;
         }
 
-    if (offset < 0) {
-        return -EIO;
-    }
-
         if (offset > len || len - offset < size) {
             return -EIO;
         }
+    }
 
     return 0;
 }

include/sysemu/block-backend.h

@@ -78,6 +78,7 @@ void blk_insert_bs(BlockBackend *blk, BlockDriverState *bs);
 
 void blk_hide_on_behalf_of_hmp_drive_del(BlockBackend *blk);
 
+void blk_set_allow_write_beyond_eof(BlockBackend *blk, bool allow);
 void blk_iostatus_enable(BlockBackend *blk);
 bool blk_iostatus_is_enabled(const BlockBackend *blk);
 BlockDeviceIoStatus blk_iostatus(const BlockBackend *blk);